Method and apparatus for analyzing and editing ECG morphology and time series

ABSTRACT

A method and apparatus for analyzing the QT interval characteristics of ECG signal data having a succession of waveforms produced by the beating of the heart. ECG signal data is obtained from a patient. The R-R interval and the QT intervals of the waveforms of the ECG signal data are determined. Waveforms of the ECG signal data having a stable heart rate are selected for use in determining the QT interval characteristics. Preferably, the waveforms selected are those having minimum R-R interval standard deviation and minimum R-R interval dispersion. The QT correction (QTc) is computed from the ECG signal data waveforms selected in the foregoing manner or on the basis of clinician editing. The R-R intervals, the QT intervals, and QTc for the heart beats of the selected waveforms are displayed for analysis and diagnosis purposes. The invention can also be used, in an analogous manner, to obtain and display other cardiac data from the ECG waveforms.

BACKGROUND AND SUMMARY

The present invention relates to a method and apparatus for analyzingand editing the morphology and time series relationships inelectrocardiographic (ECG) signals.

The heart has a right side that circulates blood to the lungs foroxygenation and CO₂ discharge and a left side that circulates oxygenatedblood to the systemic circulatory vascular field of the body. Each sidehas an atrium that receives blood during relaxation of the heart muscle(diastole) and a ventricle that discharges blood when the heart musclecontracts (systole).

ECG signal data reflects the electrical activity of the heart conductionsystem and muscle in pumping blood through the lungs and systemiccirculatory field of a subject. The signal contains a succession ofwaveforms produced by the repetitive action, or beating, of the heart.For a normal heart rate of about 60 to 80 beats per minute, thewaveforms are produced about every 0.75 to 1 second or about every 750to 1000 milliseconds.

A typical ECG waveform is shown in FIG. 1. It comprises a P wave, a QRScomplex, and a T wave. The P wave is caused by the electric potentialsgenerated when the atria of the heart depolarize before atrialcontraction occurs. The QRS complex is caused by the potentialsgenerated when the ventricles depolarize before their contraction andfeatures the prominent R peak. As the contraction and pumping action ofthe heart occurs, repolarization of the heat muscle commences, slowly atfirst and then more rapidly as the ECG waveform concludes in the T wave,in some cases, in a U wave.

In addition to the presence and shape of the components of the ECGwaveform (i.e. the morphology), the length of the components, and thespacing, or interval, between the components is useful in ECGinterpretation. Commonly used intervals shown in FIG. 1 are the P-Rinterval, QRS duration, the ST-T segment, and the QT interval. The Uwave, which is a slight depression in the S-T segment of the ECGwaveform, believed to the attributable to late repolarization of certainparts of the heart may also be used.

The use of certain drugs, or combinations of drugs, can affect the ionchannels of cardiac cells and is reflected in changes in thecharacteristics of ECG waveforms. An example of this is the use of drugssuch as some anti-depressants and anti-retrovirals, that induce aprolongation of the QT interval. This prolongation can lead to a lifethreatening arrhythmia in the form of a ventricular tachycardia(excessive rapidity) termed “torsade de pointes”, often referred tosimply as “torsade,” or TdP. Use of the QT interval is currently theonly technique approved by regulatory authorities to predict possibledrug induced TdP in clinical drug trials.

For this reason, efforts have been directed to obtaining a propermeasurement of the QT interval, as well as accurate computation of acorrection of the QT intervals of the ECG waveforms of a time series.This correction, QTc, is used to adjust the determination of the QTinterval for changes in the heart rate. Previously, the QT interval wasusually corrected based on an immediately previously occurring R-Rinterval. The R-R interval is the interval between the R peaks ofsuccessive waveforms. However, more and more research has shown thatthere could be some delay effects between R-R interval change and QTinterval change. This delay effect, often also called “hysteresis,” canbe as long as 2 minutes in some cases. But in most practical situations,only a short segment of ECG signal data, for example, 10 seconds ofdata, is available. It can make a large difference if different ECGbeats are selected for the QTc calculation if there is some type ofarrhythmia in the heart beat, as evidenced by an irregular R-R interval.Therefore, it is important to select a proper group of ECG beats for theQTc calculation within the available, short segment of ECG signal data.

Currently, in carrying out the measurement of the QT interval and QTcorrection (QTc), an amount of ECG signal is subjected to analysis usingan ECG analysis algorithm to flag those ECG waveforms in the signaldeemed suitable for QT interval and QTc measurement. The signal data isthen reviewed by a cardiologist or other clinician who decides whichwaveforms to use for the QT interval measurement and QTc computation.While this selection is designed to improve the quality of data used tocompute the QT quantities and improve the ultimate accuracy of thedetermination, at present, it is often an arduous task for theclinician.

SUMMARY OF THE PRESENT INVENTION

In determining the QT interval and the QTc, the method and apparatus ofembodiments of the present invention obtains ECG signal data anddetermines the R-R intervals and QT intervals for the waveforms in thedata. Portions of the ECG signal data exhibiting relatively stable heartrate are selected. The selection of the stable heart rate waveforms maybe based on a comparison of the standard deviation for the intervalsbetween the R features of the ECG waveforms, i.e. the R-R interval. Thedispersion between the maximum R-R interval and the minimum R-R intervalin the waveforms may also be used.

Specifically, the waveforms of ECG signal data selected are those havinga minimum standard deviation of the R-R interval and a minimumdispersion of the R-R interval. Manual selection may be aided by a noveldisplay that graphically relates the R-R intervals and the QT intervalsof the waveforms. From the selected ECG signal data, the QTc iscomputed.

Thereafter, the R-R interval, the QT interval, and the QTc are displayedfor each selected heart beat of the ECG signal data to allow analysis ofthe Q-T properties and diagnosis based thereon.

While the ECG signal data has been described as being analyzed forlonger QT intervals, as a predictor of TdP, it will be appreciated thatthe analysis may also be directed to the determination of the presenceof abnormally short QT intervals. Short QT intervals have also beenlinked to life threatening arrhythmias.

More generally, the technique of the present invention may be used toobtain and display data sets comparing two or more cardiac relatedconditions in a manner useful for diagnostic purposes.

Embodiments of the present invention will be further understood from thefollowing detailed description, taken in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing:

FIG. 1 shows electrocardiographic waveforms;

FIG. 2 is a simplified schematic drawing showing an apparatus of thepresent invention;

FIG. 3 is a flow chart showing the steps of a method of the presentinvention;

FIGS. 4A, 4B, and 4C show data displays employed in the presentinvention;

FIGS. 5A, 5B, and 5C are similar to FIG. 4 and graphically showproperties of a portion of the ECG signal data selected for use in thepresent invention to determine properties of the Q-T interval;

FIG. 6 shows a further display of the present invention;

FIG. 7 is a flow chart showing a further embodiment of the method of thepresent invention;

FIGS. 8A and 8B are tables showing electrocardiographic features thatmay be used in the method shown in FIG. 7; and

FIG. 8C is a graphical showing similar to FIGS. 4B and 5B of a displayof the present invention.

DETAILED DESCRIPTION

For use in the present invention, ECG signal data is obtained in aconventional manner by applying electrodes 10 to the body of a subject12, as shown in FIG. 2. In the apparatus of the present invention, theelectric signals in electrodes 10 are amplified in pre-amplifier 14 andare in the analog form shown in FIG. 1 comprising a series of sequentialwaveforms. The analog signals are subjected to analog-to-digitalconversion in analog/digital converter 16 and stored in the memory 18 ofcomputer 20, the operation of which using central processor unit 19containing an algorithm that carries out the method of the presentinvention. Computer 20 includes a screen 22 for displaying informationin graphic and/or textual form.

An embodiment of the method of the present invention is shown in theflow chart of FIG. 3. Following the start of the method at step 40, ECGsignal data in the form of a series of sequential waveforms is obtained,either as it is received from subject 12, or more typically from thedata stored in memory 18 of computer 20. This occurs at step 42. Thedata so obtained is shown in FIG. 4A containing two leads of ECG signaldata for a typical sampling period of about 10 seconds. In FIG. 4A, theheart beats are sequentially numbered.

The data is then analyzed to select ECG signal data waveforms having arelatively stable heart rate. Preferably, this is done by analyzing theECG signal data to determine the magnitudes of the intervals between theR peaks of successive ECG waveforms, i.e. the R-R interval. The QTinterval present in each waveform is also obtained. This is done at step44 shown in FIG. 3. The R-R interval between the R peak of a givenwaveform and that of the preceding waveform is matched to the QTinterval of the given waveform. The ECG signal data may be displayed onscreen 22 in step 46 and the R-R interval is shown in milliseconds inFIG. 4A.

To provide a graphic indication of this data, it can be displayed onscreen 22 in the manner shown in FIG. 4B in a time series relational mapthat shows the matched immediately previous R-R interval and Q-Tinterval data pairs as relational bars. The magnitude of the intervalsis indicated by the height of the vertical lines. For the R-R interval,this is lines 46 and for the Q-T interval, it is lines 48. A uniformityin the height of the R-R interval lines 46 is a visual indication of therelative stability in the heart rate of the subject. The horizontallines or ticks 50 on lines 46 indicate a certain fraction of the lengthof a line 46, for example, half or 50% of the R-R interval. This ishelpfil to a clinician in visualizing the magnitudes of the R-Rintervals and associated QT intervals. For example, a Q-T intervallonger than half the previous R-R interval is a warning sign of apossible prolonged Q-T interval.

FIG. 4A shows the ECG waveform that produced the relational map ordisplay of FIG. 4B. For beats 2 through 5, the extent of the R-Rinterval and the height of lines 46 is generally uniform, the R-Rinterval extending from 858 to 888 milliseconds. The R-R intervalbetween beats 5 and 6 is shorter, 690 milliseconds, and the R-R intervalbetween beats 6 and 7 is longer, 1072 milliseconds. This is reflected inthe differing heights of lines 46-6 and 46-7, respectively, in FIG. 4B.

FIG. 4C shows another way of displaying the waveforms for presentingvariations in their morphology and timing. In FIG. 4C, the ECG waveformdata from successive heart beats is used to produce a median or averagewaveform that is displayed on screen 22 as the darker line 52. Thewaveform 54 for each heart beat is superimposed on the median waveform52 so that atypical waveforms, and hence heart beats are readilyapparent.

In an embodiment of the method of the present invention, the selectionof the ECG signal data waveforms used to determine the QT interval andthe QTc employs the standard deviation of the R-R interval, and also thedispersion of the maximum R-R interval to the minimum R-R interval.

The standard deviation is a measure of how closely data is clusteredaround a central or average value. The simplest measure of dispersion isthe range, i.e. the difference between the maximum R-R interval and theminimum R-R interval found in a portion of ECG signal data. Otherexpressions of dispersion are available.

In an embodiment of the present invention, the ECG signal data waveformsselected are those having a minimum standard deviation STD of the R-Rinterval and a minimum dispersion of the R-R interval. The minimumstandard deviation means that the R-R interval magnitudes of theselected portion of signal data are closely clustered around a centralvalue. The minimum dispersion means that the difference between themaximum and minimum values is small. In the example of FIG. 4, theselected waveforms would be a portion including heart beat waveforms 1through 5.

The determination of the standard deviation of the R-R interval and theR-R interval maximum-minimum dispersion and the selection of a portionof the ECG signal data having minimum R-R interval standard deviationand minimum R-R interval dispersion is carried out at steps 60 and 62 ofthe method. The selected portion of the data is displayed in FIGS. 5A, Band C more particularly as heartbeat waveforms 2-4, as indicated in step64 of FIG. 3

In step 66, a clinician may manually edit the ECG signal data withrespect to the ECG signal data waveforms to be used, using either orboth of displays in the form of FIG. 4 or FIG. 5. The portion of ECGdata to be used in the further steps of the method, as selected by thecriteria of steps 60 and 62 and/or manually in step 66, is establishedat step 68 and displayed in step 70.

In step 72, the QT interval is computed for each selected waveform ofthe signal data or is obtained from the previous determination made instep 44.

In step 74, the QTc indicative of the effect of the heart rate on the QTinterval is determined for the waveforms of the selected portion of theECG signal data. A number of formulae are commonly used for thispurpose. These include the Bazett formula (QTc=QT/RR^(1/2)), theFriderica formula (QTcF=QT/RR^(1/3)), and the linear regressionequitation (QTcL=QT+0.154×[1−RR]). The linear regression equitationformula is often termed the Framingham formula with reference to theFramingham, Mass., longitudinal heart study. The computation of the QTcis carried out for each heart beat waveform in the selected portion ofECG signal data.

In step 76, data is displayed in the manner shown in FIG. 6 for theselected heart beat waveforms of the ECG signal data. As shown in FIG.6, the data is preferably graphically displayed as multiple verticalbars with time in milliseconds on the ordinate. The data displayedincludes the R-R interval 76-1, the QT interval 76-2, and the QTcorrection determined by various formulae, 76-4, 76-6, and 76-8. Thedisplay of FIG. 6, allows RR/QT/QTc trending in order to monitorreal-time dynamic change of QTc values with a view to identifyingchanges in QT interval properties indicative of potentially adverseconditions for subject 12.

In another aspect of the present invention, graphical displays of thetype shown in FIGS. 4 and 5 may be extended to more general time seriesrelational map displays to graphically show and compare two or moreaspects of the ECG waveform signal data on a time series basis for useby a clinician. For example, the graphical showing may be that of acardiac depolarization related feature along with a cardiacrepolarization feature. Or, the displayed aspects may both relate todepolarization phenomena, but one may be a ventricular depolarizationrelated feature and the other an atrial depolarization related feature.

To carry out this aspect of the present invention, following a start atstep 100 of FIG. 7, the desired features to be displayed are determinedat step 102. For example, FIG. 8A shows typical cardiac depolarizationrelated features and repolarization features that may be paired toproduce a display, such as that shown in FIG. 8C. FIG. 8B shows asimilar tabulation of ventricular depolarization related features andatrial depolarization features that may be paired. The features relatingto, for example, the QRS wave axis or the P wave axis, are determined bythe well known principles of vectorial analysis of electrocardiograms.

ECG signal waveform data is obtained in step 104. It will be appreciatedthat the sequence of steps 104 and 102 may be reversed, if desired.

If desired for the features being analyzed, a desired portion orwaveforms of the ECG signal data may be selected in step 106. Theselection may be carried out in the manner described above or in someother appropriate manner.

Thereafter, the desired features, such as a selected one of thedepolarization related features and a selected one of the repolarizationfeatures shown in FIGS. 8A are extracted from the ECG signal waveformdata at step 108. Thereafter, the data is displayed in the manner shownin FIG. 8C in step 110.

The result of the display of data in the manner shown in FIG. 8C is abeat-by-beat relational map for an ECG feature time series analysis. Therelational map can be used in the same manner as described above inconnection with the analysis of the R-R interval and the Q-T intervaldescribed using FIGS. 3 through 5.

Various alternatives and embodiments are contemplated as being withinthe scope of the following claims particularly pointing out anddistinctly claiming the subject matter regarded as the invention.

1. A method for analyzing the characteristics of ECG signal data havinga succession of waveforms produced by the beating of the heart, saidmethod comprising the steps of: obtaining ECG signal data; selecting atleast two features of the ECG signal data waveforms to be presented as adata set for heart beats of ECG signal; extracting the selected featuresfrom the waveforms of(the signal data; and displaying the features asdata sets for the heart beats.
 2. The method according to claim 1wherein the step of displaying the features is further defined asdisplaying the data sets in the form of relational bars, the amounts ofelongation of which are proportional to the magnitude of the featuresrepresented by the relational bars.
 3. The method according to claim 2wherein the extracting step is further defined as extracting thefeatures from a selected portion of the ECG signal data.
 4. The methodaccording to claim 3 wherein the selected features for the data setscomprise the R-R intervals and at least one of the QT intervals of thewaveforms of the ECG signal data and a computed QT correction (QTc). 5.The method according to claim 4 further defined as placing an indicatoron the R-R interval bars indicative of a selected fraction of the amountof elongation of the bars.
 6. The method according to claim 4 furtherdefined as computing a QT F correction (QTc) and displaying the QTc inthe data sets.
 7. The method according to claim 6 wherein the computingstep is further defined as computing multiple correction values (QTc)based on different correction formulae and the displaying step displaysmultiple correction values (QTc).
 8. The method according to claim 4wherein the extraction step is further defined as extracting thefeatures from a selected portion of the ECG signal having a stable heartrate.
 9. The method according to claim 8 further defined as usingstatistical properties of the ECG signal data to select the portion ofthe ECG signal data.
 10. The method according to claim 9 further definedas determining the standard deviation of the R-R interval and using thatdetermination to select the portion of the ECG signal data.
 11. Themethod according to claim 9 further defined as determining thedispersion of a maximum R-R interval to a minimum R-R interval and usingthe dispersion to select a portion of the ECG signal.
 12. The methodaccording to claim 8 further defined as determining the dispersion of amaximum R-R interval to a minimum R-R interval and using the dispersionand the standard deviation to select a portion of the ECG signal. 13.The method according to claim 2 wherein the extracting step is furtherdefined as extracting the features from a portion of the ECG data and asmanually editing the ECG signal data to determine, at least in part, theportion of the ECG signal data from which the features are extracted.14. The method according to claim 13 further defined as manually editingthe ECG signal data using the relational bars to determine, at least inpart, the portion of the ECG signal data from which the features areextracted.
 15. The method according to claim 9 further defined asmanually editing the ECG signal data using the relational bars todetermine, at least in part, the portion of the ECG signal data fromwhich the features are extracted.
 16. The method according to claim 12further defined as manually editing the ECG signal data using therelational bars to determine, at least in part, the portion of the ECGsignal data from which the features are extracted.
 17. The methodaccording to claim 13 further defined as determining a median waveformfor a heart beat and as comparing the waveforms of the signal data tothe median waveform in manually editing the ECG signal data.
 18. Themethod according to claim 2 wherein the selected features of the datadisplayed in the form of relational bars are a depolarization relatedfeature and a repolarization related features.
 19. The method accordingto claim 18 wherein the depolarization related feature is a selected oneof heart rate, the R-R interval, the QRS axis, the QRS amplitude, andthe Q wave duration.
 20. The method according to claim 18 wherein therepolarization related feature is a selected one of the Q-T interval,the T wave axis, the Q-T interval dispersion, and the T wave amplitude.21. The method according to claim 19 wherein the repolarization relatedfeature is a selected one of the Q-T interval, the T wave axis, the Q-Tinterval dispersion, and the T wave amplitude.
 22. The method accordingto claim 2 wherein the selected features of the ECG signal datadisplayed in the form of relational bars are a ventricular relatedfeature and an atrial related feature.
 23. The method according to claim22 wherein the features related to ventricular and atrialdepolarization.
 24. The method according to claim 23 wherein the featureis a ventricular related depolarization feature comprising one of theR-R interval, the QRS axis, the QRS amplitude, and the Q wave duration.25. The method according to claim 23 wherein the feature is an atrialdepolarization feature comprising one of the P-R interval, the P waveaxis, the P-R dispersion, and the P wave duration.
 26. The methodaccording to claim 24 wherein the feature is an atrial depolarizationfeature comprising one of the P-R interval, the P wave axis, the P-Rdispersion, and the P wave duration.
 27. A method for analyzing thecharacteristics of ECG signal data having a succession of waveformsproduced by the beating of the heart to determine the QT intervalproperties of the ECG signal data, said method comprising the steps of:obtaining ECG signal data; determining the R-R intervals and related QTintervals of the waveforms of the ECG signal data; selecting a portionof the ECG signal data having a stable heart rate; computing a QTcorrection (QTc) from the selected waveforms of the ECG data; anddisplaying the R-R intervals and at least one of the QT intervals, andthe QTc correlations in the form of relational bars for the heart beatsof the selected waveforms.
 28. The method according to claim 27 whereinthe step of selecting the waveforms of the ECG signal data is furtherdefined as determining the standard deviation of the R-R interval andthe dispersion of a maximum R-R interval to a minimum R-R and using thestandard deviation and dispersion to select the waveforms of the ECGsignal data.
 29. Apparatus for analyzing the characteristics of ECGsignal data having a succession of waveforms produced by the beating ofthe heart, said apparatus comprising: means for selecting at least twofeatures of the ECG signal data waveforms to be presented as a data setfor heart beats of ECG signal; means for extracting the selectedfeatures from the waveforms of the ECG signal data; and means fordisplaying the features as data sets for the heart beats in the form ofrelational bars, the amounts of elongation of which are proportional tothe magnitude of the features represented by the bars.
 30. The apparatusaccording to claim 29 wherein said means for extracting extracts thefeatures from a selected portion of the ECG signal data.
 31. Theapparatus according to claim 29 wherein said means for selecting selectsfeatures for the data sets comprising the R-R intervals and the QTintervals of the waveforms of the ECG signal data.
 32. The apparatusaccording to claim 31 wherein said means for displaying places anindicator on the R-R interval bars indicative of a selected fraction ofthe amount of elongation of the bars.
 33. The apparatus according toclaim 29 further including means for computing a QT correction (QTc) andsaid displaying means displays the QTc in the data sets.
 34. Theapparatus according to claim 33 wherein said means for computing isfurther defined as computing multiple correction values (QTc) based ondifferent correction formulae and said displaying means displaysmultiple correction values (QTc).
 35. The apparatus according to claim30 further defined as including means for determining the standarddeviation of the R-R interval and said selection means uses thatdetermination to select the portion of the ECG signal data.
 36. Theapparatus according to claim 30 further defined as including means fordetermining the dispersion of a maximum R-R interval to a minimum R-Rinterval and said selecting means uses the dispersion to select aportion of the ECG signal.
 37. The apparatus according to claim 35further defined as including means for determining the dispersion of amaximum R-R interval to a minimum R-R interval and using the dispersionand the standard deviation to select a portion of the ECG signal. 38.The apparatus according to claim 29 wherein said means for extractingextracts the features from a portion of the ECG data and said apparatusincludes means for manually editing the ECG signal data to determine, atleast in part, the portion of the ECG signal data from which thefeatures are extracted.
 39. The apparatus according to claim 38 furtherdefined as including means for determining a median waveform for a heartbeat and for comparing the waveforms of the signal data.
 40. Theapparatus according to claim 29 wherein the selected features of thedata are a depolarization related feature and a repolarization relatedfeatures.
 41. The apparatus according to claim 29 wherein the featuresof the ECG signal data are a ventricular related feature and an atrialrelated feature.
 42. Apparatus for analyzing the characteristics of ECGsignal data having a succession of waveforms produced by the beating ofthe heart to determine the QT interval properties of the ECG signaldata, said apparatus comprising: means for determining the R-R intervalsand related QT intervals of the waveforms of the ECG signal data; meansfor selecting waveforms of the ECG signal data having a stable heartrate; means for computing a QT correction (QTc) from the selectedwaveforms of the ECG data; and means for displaying the R-R intervalsand at least one of the QT intervals, and the QTc correlations for theheart beats of the selected waveforms in the form of relational bars.43. The apparatus according to claim 42 further including means fordetermining the standard deviation of the R-R interval and thedispersion of a maximum R-R interval to a minimum R-R and using thestandard deviation and dispersion to select the waveforms of the ECGsignal data.